where E is the Young’s modulus of the material, a half the indentation diagonal, l the crack length, and k the dimensionless constant determined experimentally. The measured toughness for the composite is 9.7 ± 0.3 MPa m1/2. This value is much higher than that(4.8 ± 0.5 MPa m1/2) of the SiCw–ZrO2(with 6 mol% Y2O3) composites made by hot pressing; but lower than that (16.3 ± 1.1 MPa m1/2)of the SiCw/ZrO2(with 2 mol% Y2O3,) composites [8,9]. It was reported that the high Y2O3content promoted the formation of interfacial amorphous layers, which strengthened the interfacial bonding and resulted in low fracture toughness [8,9]. The cracks were closely examined under SEM (Fig. 5b and d). Crack deflection,transgranular fracture, and bridging can be clearly observed along the crack wake, which, together with transformation toughening are likely responsible for the observed toughness [8,9,28].